1. Field of the Invention
The present invention relates to an ink jet printer, and in particular, to improvements in an ink supply system that supplies ink to a printing head.
2. Description of the Related Art
Inkjet printing apparatuses which form an image on a printing medium by depositing ink to the printing medium using an inkjet printing head include that which form an image by ejecting ink while moving a printing head relative to a printing medium and that which form an image by ejecting ink while moving a printing medium relative to a fixed printing head conversely.
There are two general types of methods of supplying ink to a printing head used in such an inkjet printing apparatus. One is a type in which a supply system is configured such that an amount of ink is always or continuously supplied to a printing head according to the amount of ink ejected (hereinafter referred to as a continuous supply type), and the other is a type in which a printing head is provided with a reservoir (sub-tank or second ink tank) for reserving a predetermined amount of ink and in which a supply system is configured such that ink is supplied to the reservoir from an ink supply source (main tank or first ink tank) at appropriate timing or intermittently (hereinafter referred to as an intermittent supply type).
The continuous supply type is further categorized into two types, for example, when it is used in an inkjet printing apparatus of a type referred to as a serial type in which a printing head is scanned back and forth in predetermined directions relative to a printing medium and in which the printing medium is transported in a direction substantially orthogonal thereto to form an image. One is a type referred to as an on-carriage type in which ink is supplied by integrally or detachably attaching an ink tank to a printing head that is carried and moved back and forth (main scanning) by a carriage. The other is a tube supply type in which an ink tank that is separate from a printing head carried on a carriage is fixedly installed in a part of a printing apparatus other than the printing head and in which the ink tank is connected to the printing head through a flexible tube to supply ink. In some of the latter type, a second ink tank that serves as an intermediate tank between an ink tank and a printing head is mounted on the printing head or the carriage.
When an on-carriage type structure is adopted, there are limits on the project area in a direction perpendicular to the main scanning direction and volume of members that move with a carriage (a printing head and an ink tank undetachably or detachably integrated with the same). Therefore, only an ink tank having a very limited capacity can be used when a small-sized printing apparatus, especially, a portable printing apparatus is to be formed. This results in very frequent replacement of the printing head integral with the ink tank or the ink tank alone, which has been problematic from the viewpoint of operability and running cost.
When a tube supply type structure is adopted, although members that move with a carriage during main scanning can be made compact to some degree, it is difficult to make the apparatus as a whole compact because a space is required for a tube member to move to follow up the carriage, the tube member coupling a printing head on the carriage and an ink tank located outside the carriage to supply ink. Further, the recent trend is that a carriage is scanned at a high speed to accommodate increases in the speed of printing operations, and resultant severe rocking of a tube that follows the carriage results in changes in the pressure of ink in an ink supply system for the printing head. It is therefore required to provide various complicated pressure buffering mechanisms in order to suppress pressure changes, it has been difficult to achieve a size reduction in this respect too.
On the contrary, in the case of the intermittent supply method that is used for serial type inkjet printing apparatus for example, a relatively small second ink tank and printing head are provided on a carriage; a relatively large first ink tank is provided in a part other than the carriage of the printing apparatus; and a supply system is configured such that ink is supplied from the first ink tank to the second ink tank at appropriate timing. A structure is also employed in which the ink supply system between the first and second ink tanks is spatially separated or the ink channel is blocked with a valve during main scanning to achieve fluid isolation between the first and second ink tanks (For example Japanese Patent Application Laid-open Nos. 5-238016 (1993) and 2000-86819). Basically, this makes it possible to solve various problems attributable to the size of moving members as described above such as an ink tank and the rocking of a tube that have limited efforts to achieve a small size in the case of the continuous supply type.
With such an intermittent supply method, a configuration is employed in which the printing head is moved to an area outside a scanning range for printing so that a first and second ink tanks are fluid-coupled together in this area. Thus, a joint portion for this fluid communication must be arranged outside the scanning range for printing. This restricts a reduction in the size of the ink jet printer.
In contrast, Japanese Patent Application Laid-open No. 2000-334982 discloses a configuration in which a joint portion that fluid-couples a first and a second ink tanks together is arranged above the scanning range of a printing head. In this configuration, the printing head is rotated to this joint position for fluid communication. This configuration utilizes spaces more effectively than the configuration in which the joint portion is arranged in the area outside the scanning range for printing. This facilitates a reduction in the size of the ink jet printer.
With the ink jet printing method, ink is ejected from ejection openings to a printing medium. In recent years, an increase in printing resolution or quality has been strongly desired. Accordingly, the size of each ejection opening has been increasingly reduced and a correspondingly reduced amount of ink has been ejected from the ejection openings. Thus, it has been desirable to deposit very accurately ink to a printing medium at specific positions. To achieve this, the clearance between the ejection openings and the printing medium must be rigidly managed.
However, all the conventional ink jet printers based on the intermittent supply method are based on the movement of the printing head to the joint position, where the fluid communication takes place. Further, a relatively strong force is required to connect and remove the ink tanks to and from the joint portion. Consequently, in the configuration in which the printing head is moved for such connection or removal, the mounting accuracy of the printing head may decrease, the position of which must be rigidly managed with respective to the printing medium.